Among the prior art systems for controlling transmission speed by utilizing road data stored in a navigation system, Japanese patent publication No. 6-272753 has proposed a control device that controls transmission speed based on various items of information, including vehicle speed variation and degree of accelerator opening, that can be detected by sensors, and map information read out from the navigation system. Such a device controls a vehicle to have an optimum transmission speed in conformity with variations in the drive conditions.
With the above-described prior art transmission control devices, road data is obtained from the navigation system in order to determine a transmission speed in conformity with the conditions of the road on which the vehicle is currently travelling. However, all operating parameters used in such prior art control merely indicate the current drive conditions. With the prior art systems it is not at all possible to control the transmission in anticipation of future change in the drive conditions.
For example, when the vehicle goes up a slope, the prior art control device can not change transmission speed until the vehicle ascends to a certain level that results in a change in actual vehicle speed or in the degree of accelerator opening. Moreover, this control tends to follow the road information but neglects the driver's will or intention. More specifically, the transmission speed control based on the road information has a good adaptability to surrounding conditions, but tends to force a perfunctory control on the driver.
Further, when the vehicle is about to enter a winding stretch of road, the transmission should be controlled to keep gear ratios within a relatively low range. However, in accordance with the prior art control, the transmission speed would be upshifted in response to release of the accelerator pedal, which reduces drive power and thus does not allow smooth acceleration in accordance with the driver's intention. When the vehicle enters a curve with the accelerator pedal not depressed, it is difficult to provide appropriate transmission control in accordance with the driver demands.
Accordingly, it is an object of this invention to achieve vehicle control that conforms well to the driver's intention, and more particularly to provide a vehicle control system that achieves more favorable control in response to a decelerating operation by the driver.